The present invention relates to a process for separating radioactive iodine compounds contained in a liquid waste by precipitation. More particularly, it relates to a process wherein iodates contained in a liquid waste are reduced and then precipitated with silver nitrate.
The process of the present invention is applicable to the disposal of liquid waste discharged from, e.g., nuclear power plants, reprocessing plants and various nuclear energy research facilities.
A liquid waste discharged from nuclear facilities contains radioactive iodine compounds (mainly comprising molecular iodine, iodates and iodides). Known methods for the disposal of such radioactive iodine compounds in the liquid waste include (1) solidification, (2) ion exchange resin method, and (3) coagulating sedimentation method.
The solidification method (1) is a method of confining the iodine compounds in a solidified material such as asphalt. The iodine compounds flowing into an off-gas system are adsorbed on a silver/zeolite filter. According to the method (1), molecular iodine (I.sub.2) and/or organoiodine compounds tend to be released by the action of heat generated during solidification and iodide ion (I.sup.-) may be oxidized into volatile molecular iodine (I.sub.2). Further, the silver/zeolite filter is poorly effective in capturing the iodine compounds in some cases. The ion exchange resin method (2) is a method of passing the liquid waste through an ion exchange resin to adsorb the radioactive iodine compounds on the resin, thereby separating the compounds. However, the method (2) has a problem that the separation of the iodates is difficult. The coagulating sedimentation method (3) is a method of adding silver nitrate to the liquid waste to precipitate the iodine compounds. The method (3) also has a problem that the precipitation and separation of the iodates is difficult.
The main chemical forms of radioactive iodine contained in the above-described liquid waste are iodate ion (IO.sub.3.sup.-) and iodide ion (I.sup.-). The iodate ion has a possibility that it cannot be sufficiently separated by the ion exchange resin method or the coagulating sedimentation method according to the prior art but will be discharged into the sea. Alternatively, the residual iodate ion may be converted into volatile iodine in the subsequent solidification step to be discharged into the atmosphere. On the other hand, the iodide ion is in danger of being evaporated during the treatment of the liquid waste or being converted into a volatile chemical form by the action of heat or air and discharged into the atmosphere.